I'd like to run some bitcoin daemons unattended, and have them generate the blocks but with a predefined public key, rather than a fresh one, so i don't have to check them manually / transfer coins manually.

I have made do for the moment with a cron job that attempts to send 50 every hour

The JSON-RPC interface can be used to query the balance and send it. Try something similar to this.. this will also start bitcoind if it's not running already. You can run this every few hours from cron.